My friend has a 99 WR400 that has a problem. I know very little about thumpers. Here's the problem. The bike will not start without replacing the spark plug every time he rides. Once a new spark plug is installed, the bike runs great. During the ride, he can shut the bike off and restart it without a problem as long as he does it in a short period of time. If the bike sits for more than 15-20 minutes, however, the sparkplug must be replaced again. Please... any ideas for the mechanically retarted?